1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and a method of generating an ultrasonic image capable of correcting the blur of an image due to the movement of an object to be examined or the movement of an operator's hand holding an ultrasonic probe.
2. Description of the Related Art
In recent years, a ‘contrast echo method’ of injecting an ultrasonic contrast medium into a vein has been used for ultrasonic diagnosis. The contrast echo method is a method of injecting the ultrasonic contrast medium into the vein, increasing a bloodstream signal, and evaluating the amount of the bloodstream in the heart or the liver. In many cases, minute bubbles of the contrast medium (contrast medium bubbles) serve as a reflective source. From the viewpoint of the property of bubbles, which is a delicate material, bubbles are broken by the mechanical operation of ultrasonic waves having a general diagnostic level, and as a result, the signal intensity from the scanning surface is lowered. Therefore, in order to observe the dynamic aspect of the reflux of the bloodstream in real time, a method of reducing the breaking of bubbles due to scanning, such as a method of forming an image by transmitting low sound pressure ultrasonic waves, is needed.
The following method has been developed considering the property of a material, such as the breaking of bubbles. That is, a low sound pressure ultrasonic wave is transmitted to observe the movement of bubbles filled in the scanning surface, a high sound pressure ultrasonic wave is transmitted to the scanning surface (strictly, in an object to be radiated) to break the bubble, and a low sound pressure ultrasonic wave is radiated to the scanning surface to observe the movement of bubbles flowing into the scanning surface. This method is called a ‘replenishment method’ (for example, see JP-A-11-155858).
The contrast medium bubbles are characterized in that it can form an image of a minute bloodstream which cannot be imaged by a color Doppler method. The imaging of the minute bloodstream is abnormal since few bubbles exist in the minute bloodstream. Therefore, a method of superposing the images of the contrast medium bubbles that appear to be abnormal to form a clear image of the structure of a minute blood vessel has been developed. This method is called a ‘micro flow imaging (MFI) method’ (for example, see JP-A-2004-321688).
In the MFI method, it is necessary to superpose ultrasonic images corresponding to a plurality of frames. Therefore, a person to be examined needs to hold his/her breath for a predetermined amount of time, or an operator needs to fix an ultrasonic probe for a predetermined amount of time.
However, it is difficult for the person to be examined to hold his/her breath for a predetermined amount of time, and it is not easy for an operator unaccustomed to the MFI method to fix the ultrasonic probe for a predetermined amount of time. Therefore, it is considerably effective to correct the blur of an ultrasonic image, in order to improve the quality of a displayed image in the MFI method.
A technique for correcting the blur of a continuous series of frames, such as moving pictures, has been used for video cameras on the market. A method of calculating relative motion vectors between frames of image data is used as a representative example. In this method, image data corresponding to one frame is divided into a plurality of areas, and the motion vector between the frames in each area is calculated on the basis of the correlation between the image data in the areas. When a plurality of motion vectors calculated for each area are used to correct the display position of the image data, it is possible to improve the visibility of a moving picture since the display position of an object does not vary even when a user's hand holding the camera is jiggled more or less.
A panoramic imaging technique, which is similar to a technique for correcting the blur of an ultrasonic diagnostic image, has been known. In this technique, ultrasonic images corresponding to a plurality of frames are acquired while moving an ultrasonic probe little by little, and overlapping portions of the ultrasonic images are combined with one another to form a wide still picture, such as a panoramic photograph. In the panoramic imaging technique, since it is necessary to combine overlapping portions of the ultrasonic images corresponding to a plurality of frames, relative motion vectors between frames are needed, similar to the ultrasonic images.